1. Field of the Invention
The present invention relates to a display device including a light emitting element and a method for inspecting the display device.
2. Description of the Related Art
A light emitting element has a self-light emitting property; therefore, it is superior in visibility and viewing angle. Accordingly, a light emitting device including a light emitting element has attracted attention, along with a liquid crystal display device (LCD).
An organic EL element in which a plurality of organic layers is interposed between an anode and a cathode is given as an example of the light emitting element. The organic layers specifically include a light emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, an electron transport layer, and the like. Such an organic EL element can be made to emit light by making a potential difference between a pair of electrodes.
In an attempt to put the light emitting device into practical use, an extension of the life of the organic EL element is said to be an important issue. The deterioration of the organic layers over time causes a decrease in luminance of the organic EL element. The rate of deterioration over time depends on material properties, a sealing method, a driving method of the light emitting device, and the like. The organic layers are particularly susceptible to moisture, oxygen, light, and heat; therefore, these factors also promote the deterioration over time.
In addition, in an attempt for practical use, it is desired that the amount of current flowing through the organic EL element be constant regardless of temperature. Even if a voltage applied between the electrodes of the organic EL element is constant, the current flowing through the light emitting element increases as the temperature of the organic layer becomes higher. In other words, when the display device is driven with constant voltage, luminance change and chromaticity deviation occur in accordance with temperature change. For such a light emitting device including an organic EL element, a technique for maintaining constant luminance of the light emitting element regardless of ambient temperature is proposed (for example, see Reference 1: Japanese Published Patent Application No. 2002-333861).